Magnesium and magnesium alloy have superior characteristics as a light-weight structural member, since they are light in weight (specific gravity: 1.7) and large in specific strength. However, they have not been used widely so far, since the cost related to the production (or the energy necessary for the production) and the like are relatively high. Upon producing magnesium and magnesium alloy, since magnesium and magnesium alloy molten at high temperature react vigorously with oxygen in the air and combust, it is necessary to have special facilities and techniques for their melting and casting. As one of ignition proof effect provisions for magnesium alloy, it has been tried to provide metal itself with ignition proof effect by adding calcium (Ca), beryllium (Be) or the like, but it is not necessarily sufficient. Besides, a method of pouring a protective flux onto a molten metal, a method of covering a metal surface with an inert gas such as helium, argon or nitrogen gas, or a method of covering it with a protective gas that forms a protective film on the metal surface has been tried, in order to prevent an abrupt oxidation (combustion) of molten magnesium and magnesium alloy.
As a protective gas in the magnesium and magnesium alloy production steps, sulfur dioxide (SO2) has been historically used many times, since it has a low price and is easily available. However, it is limited in use environment and equipment, since it is high in bad odor, metal corrosiveness and toxicity. In place of this, sulfur hexafluoride (SF6), which is low in toxicity and odorless, has been widely used, since it has no flammability and an advantageous effect at a relatively low concentration [Non-patent Publication 1]. SF6, however, has a global warming potential (GWP) that is about 24,000 times that of carbon dioxide (CO2) and furthermore has a very long atmospheric lifetime of 3,200 years. Therefore, it is an object of limitation as a warming substance in Kyoto Protocol. Magnesium and magnesium alloy become energy-saving materials, since they contribute to weight reduction when used as structural members of automobiles and the like. However, SF6 ejected during the production is a substance that has a great impact on global warming, thereby canceling out the energy-saving part. Thus, there is a strong demand for the development of a protective gas alternative to SF6.
Various fluorine-series compounds have been proposed as protective gases alternative to SF6. For example, in Patent Publication 1, Japanese Patent Application Publication 2002-541999, difluoromethane (HFC-32), pentafluoroethane (HFC-125), 1,1,1,2-tetrafluoroethane (HFC-134a), difluoroethane (HFC-152a), heptafluoropropane (HFC-227ea), methoxy-nonafluoroethane (HFE-7100), ethoxy-nonafluoroethane (HFE-7200), and dihydrodecafluoropropane (HFC-43-10me) are cited. Of these, a combination of HFC-134a and dry air is recommended as a preferable composition. Furthermore, in Patent Publication 2, US Patent Application Publication 2003/0034094; Patent Publication 3, US Patent Application Publication 2003/0164068; and Patent Publication 4, Japanese Patent Application Publication 2004-276116, perfluoroketones, ketone hydrides and their mixtures are cited as protective gases. Specifically, pentafluoroethyl-heptafluoropropylketone (C2F5(CO)C3F7) is shown as an example. Furthermore, boron trifluoride (BF3), silicon tetrafluoride (SiF4), nitrogen trifluoride (NF3), and sulfuryl fluoride (SO2F2) are cited in Patent Publication 5, U.S. Pat. No. 1,972,317.    Non-patent Publication): J. W. Fruehling, J. D. Hanawalt, Trans. AFS 77, 159 (1969)    Patent Publication 1: Japanese Patent Application Publication 2002-541999    Patent Publication 2: US Patent Application Publication 2003/0034094    Patent Publication 3: US Patent Application Publication 2003/0164068    Patent Publication 4: Japanese Patent Application Publication 2004-276116    Patent Publication 5: U.S. Pat. No. 1,972,317